One way to increase the performance of a personal computer system is to replace the microprocessor in the system with a higher speed microprocessor having local high speed devices for example cache memory or RAM memory that can be accessed by the microprocessor. Peripheral circuits such as direct memory access (DMA), floppy and hard disk controllers, video display and interrupt controllers, real time clock, timers and read only memory (ROM) of the computer system may be accessed at the original rate using the original microprocessor clock. Although replacing all of the peripheral circuits in the computer system with circuits which can operate at the new higher speed would yield maximum performance, the cost would be high and many of the higher speed circuits would have only small performance increases. Therefore it is most cost effective to replace only those pans of the computer system which give the highest increase in performance for the cost.
Some previous performance increasing solutions have used a higher speed microprocessor with cache or local RAM memory operating from a local high frequency free-running oscillator. To perform data transfers with the original computer system, the signals going to the system are first synchronized to the original microprocessor clock and then the returning signals are synchronized to the new higher frequency microprocessor clock. However, this double synchronization process for each access to the slower peripheral circuits can impose a severe performance penalty. Providing the ratio of new to original microprocessor clock rate combined with the hit ratio of the high speed local memory is sufficient, the performance will increase, but the increase would be greater if some of the synchronizing delays can be eliminated. In cases where the clock ratio is small and/or the local memory hit ratio is small, a reduction in performance compared to the original computer system can be expected due to the synchronizing delays.